A conventional boiling water reactor (BWR) includes a pressure vessel containing a nuclear reactor core submerged therein in water. Disposed above the core is a steam separator assembly having standpipes or risers extending upwardly from the core for channeling thereto a steam/water mixture, with the steam being separated from the water in the separator assembly.
Disposed above the core in the region of the steam separator assembly is a conventional feedwater sparger which typically includes one or more arcuate portions having injector nozzles for injecting into the pressure vessel when desired relatively cold feedwater for maintaining proper water level in the reactor. The feedwater is conventionally channeled to the pressure vessel through one or more feedwater inlet pipes which are typically welded to corresponding feedwater inlet nozzles integrally formed with the pressure vessel and to which are disposed in flow communication the feedwater spargers inside the pressure vessel. Since the feedwater is relatively cold, and the pressure vessel including the feedwater inlet nozzle is relatively hot, a thermal sleeve is provided inside the inlet nozzle for channeling the feedwater from the inlet pipe directly to the feedwater sparger and thereby thermally insulating the feedwater nozzle from the feedwater. In this way, thermal fatigue of the feedwater nozzle itself is minimized for ensuring a suitably long life of the pressure vessel.
Since a typical BWR may operate for twenty or more years for producing steam to power a steam turbine-generator, for example, the thermal sleeve within the feedwater nozzle may develop leaks of the feedwater which can flow around the outer surface of the thermal sleeve and in direct contact with the feedwater nozzle and thereby provide undesirable cooling thereof. The thermal sleeve is typically welded at one end to the feedwater sparger, with its opposite end being inserted through the feedwater nozzle and in an interference fit with a tubular transition piece which is also known as a safe end. The pressure vessel and the integral feedwater nozzle are typically formed of relatively low alloy steel, and the safe end is typically welded to the feedwater nozzle and is formed of a more expensive alloy steel for forming an effective interference fit seal with the distal end of the thermal sleeve. The feedwater inlet pipe is in turn typically welded to the safe end for completing the flow passage for channeling the feedwater through the wall of the pressure vessel. After initial welding of the safe end to the feedwater nozzle, the feedwater nozzle requires post-weld heat treatment for relieving heat-affected-zone residual stresses therein resulting from the welding operation. The resulting assembly including the safe end provides a relatively long lasting joint for channeling the feedwater into the pressure vessel. However, corrosion or erosion, or both, at the interference fit between the thermal sleeve and the safe end after extended operation of the reactor in service, can result in leakage through the interference fit.
In one conventional repair method used prior to the initial commercial operation of the reactor, the safe end is removed and replaced with a new safe end having a stainless steel tubular inlay. The original thermal sleeve is also removed and replaced with a new double piston ring thermal sleeve, also referred to as a triple thermal sleeve. And, the original feedwater sparger, which originally is welded to the original thermal sleeve, must also be removed and replaced with a new feedwater sparger welded to the replacement triple thermal sleeve, which is accomplished outside the reactor vessel on the refueling floor.
In the event a leak develops after initial commercial operation of the reactor, another conventional repair method includes machining away a portion of the inner surface of the existing safe end for providing an interference fit with a replacement thermal sleeve. As in the above method, both the original feedwater sparger and the original thermal sleeve welded thereto must again be removed and replaced with a new feedwater sparger and a similar triple thermal sleeve as described above. This alternate repair method is conducted from inside the reactor pressure vessel with access through the feedwater nozzle.
Since a typical BWR pressure vessel includes several feedwater nozzles, all of which are repaired even if only one is found to be leaking, the cost associated with the repairs is quite substantial. Furthermore, the time required to effect the repairs is considerable, which subjects repair personnel to radiation exposure during the repair procedures.